The present invention proposes a new brush holder for metal fiber and metal foil brushes. It is designed to guide the brush in axial direction as it wears even while a constant light brush pressure is exerted and a large current is conducted to or from the brush at very low electrical resistance. The invention is depicted in FIGS. 1A to 1E and is called the xe2x80x9ctubular brush holder.xe2x80x9d
In the version of FIGS. 1A and 1B, fiber brush 27 is conductively fastened to metal baseplate 28 and is pushed forward in electrically conductive brush box 29 by constant force spring 37 so as to load brush 27 against rotor 2 with the desired brush force. At least one of the four long sides of the inside of brush box 29 is lined in an electrically conductive manner with resilient multi-contact metal material 47, i.e., in the form of metal velvet, metal felt, strands of fine metal fibers, metal fibers combined in the manner of textiles, e.g., through weaving or knitting or any other. At least one metal guide 38 is rigidly, electrically conductive attached to base plate 28 in such a manner that it is parallel to at least one side of the brush box that is lined with resilient multi-contact metal material 47 and is disposed so that it guides fiber brush 27 along the inside of brush box 29 while the brush wears.
Low-resistance electrical contact is established between the at least one guide 38 and at least one multi-contact metal material by means of at least one compression spring 54 (in this case depicted as helical springs 54(1) and 54(2) extending between guide 38 and a thin low-friction plate 39 that is flexibly hinged to baseplate 28 disposed so that it is parallel to the at least one metal guide 38. Teflon may be a particularly suitable material for hinged low-friction plate 39. For further stabilization of the brush motion, the remaining inner surfaces of the brush box may be provided with low-friction liner or the edges of base plate 28 may be lined with a low-friction material such as Teflon. Such a liner is indicated by number 56.
The spring action between guide 38 and hinged low-friction plate may be provided by at least one conventional spring, e.g., a helical spring, or by at least one constant force spring. The spring force is adjusted to compare with the brush force exerted on fiber brush 27 by means of constant force spring 37. The friction force due to the described elastic compression of the multi-contact metal material 47 and metal guide 38 will reduce the brush force on fiber brush 27 by about 30% of the force imposed by constant force spring 37, for the reason that the coefficient of friction between multi-contact metal material and smooth metals is in the range of 0.2 to 0.3.
The contact resistance between brush box 29 and fiber brush 27 via the resilient multi-metal material 47 will be about one half of the electrical resistance between the brush and rotor 2 since static multi contacts have about one half of the resistance of similar sliding contacts under same pressure (compare C. M. Adkins III and D. Kuhlmann-Wilsdorf, xe2x80x9cDevleopment of high-performance Metal Fiber Brushes IIxe2x80x94Testing and Propertiesxe2x80x9d, Electrical Contactsxe2x80x941979 (Proc. Twenty-Fifth Holm Conf. On Electrical Contacts, III. Inst. Techn., Chicago, Ill., 1979), pp. 171-184, the entire contents of which is incorporated by reference herein.), provided that the surfaces are clean. This is a requirement that must be fulfilled, either by operating in a protective atmosphere such as humidified CO2 or making the contact surface between multi contact metal material and slider of noble metal or plating with a noble metal.
In cases of high packing density of brush holders, e.g., as may be the case in homopolar motors, the outside of the brush box should be coated with a nonconductive paint or lacquer, e.g., red stop-off lacquer.
For a particular application, the tubular brush holder designed in FIGS. 1A and 1B may be too costly or requires too much space. In fact restricted space for bushes is a perennial problem for homopolar motors since their appeal derives from their potentially large power to weight ratio and, due to the typically low voltage per current turns, they require large numbers of brushes. FIGS. 1C to 1E therefore show simplifications of the concept of the tubular brush holder according to the present invention that are designed both to save space and cost.